With all solar collectors there exists the potential for the collectors to reach very high temperatures, particularly during periods where there is little or no heat removal from the collectors. For example, in solar heating systems that rely on the circulation of a heat transfer fluid through absorbers to remove heat from collectors, such as solar hot water systems, the collectors can reach high temperatures during power failures when circulation of the heat transfer fluid stops, or during prolonged periods of little or no hot water consumption. Under these conditions, solar collectors may reach “stagnation” temperatures exceeding 170° C. In addition to the possibility of damage to collector components, exposure to such high temperatures can rapidly degrade or even boil the heat transfer fluid. Also, excessive pressures will result in the solar collector heat transfer loop as a result of the high stagnation temperatures.
The problem is particularly acute in climates where there is a potential for the occurrence of freezing temperatures during part of the year. Solar heating systems designed for these climates typically use an anti-freeze heat transfer fluid to transport heat from the solar collectors to a load. Commonly used anti-freeze fluids are propylene-glycol/water mixtures, which are subject to deterioration at elevated temperatures (e.g., greater than approximately 120° C.). Elevated collector temperatures may cause this heat transfer fluid to become corrosive, resulting in accelerated fouling and corrosion of the solar collector components and associated system components.
In addition to these reliability issues, elevated solar collector temperatures in solar hot water systems may also result in scalding temperatures in the hot water storage—a potentially dangerous situation for users of the system. To avoid this potentially harmful situation, it is possible to shut down the circulation of heat transfer fluid through the solar collectors (e.g., by shutting down the circulation pump) when the thermal storage reaches a high temperature. While this reduces the potential for scalding, it only aggravates the high stagnation temperature problem.
There have been proposed a number of designs for avoiding excessive temperatures in solar collectors. For example, U.S. Pat. No. 4,150,659 to Buckley, U.S. Pat. No. 4,219,009 to Palmer, and U.S. Pat. No. 4,503,840 to Chertok each disclose a solar collector in which venting of the space between the absorber and the glazing is provided, by disposing vents with thermally-actuated dampers on the upper surfaces or ends of the collectors. U.S. Pat. No. 4,046,134 to Scott proposed a solar collector in which the space between the glazing and the absorber was vented by raising and lowering the glazing above the rest of the collector, using a thermally-actuated mechanism. U.S. Pat. No. 5,404,867 to Rich disclosed a solar collector in which the space between the glazing and the absorber was vented by providing a glazing that flexed when a certain high temperature was reached, thereby providing ventilation at the perimeter of the glazing. U.S. Pat. No. 4,226,225 to Niedermyer disclosed a solar collector in which the space on either side of the absorber was vented by thermally-actuated vents disposed on the sides of the collector. U.S. Pat. No. 4,422,443 described a solar collector with double glazing, in which the space between the glazing was vented by thermally-actuated vents disposed on the sides of the collector. U.S. Pat. No. 4,237,865 to Lorenz disclosed a solar heating panel having an air channel, for convective air heating of a building. Thermally-actuated vents disposed on the front of the panel provided venting of the channel. However, a problem with all of these previous designs is that the vents are provided on the top or sides of the collector, which allows for debris and moisture to enter the collector and accumulate on the inner surface of the glazing and on the surface of the absorber, lowering efficiency and increasing maintenance costs of the collector. Further, such placement of the vents renders them vulnerable to weather, reducing their reliability.